NASA's Kepler spacecraft, patiently measuring the light of distant suns to find the tell-tale dimming caused by the passage of unseen planets, has discovered a solar system 210 light years from Earth with the smallest planet yet found orbiting another sun-like star, NASA announced Wednesday.

The artist's concept depicts the new planet dubbed Kepler-37b. The planet is slightly larger than our moon, measuring about one-third the size of Earth. Credit: NASA/Ames/JPL-Caltech

Dubbed Kepler-37b, the planet is smaller than Mercury and only slightly larger than the moon. The Kepler data also revealed two other planets, one slightly smaller than Earth and one twice as large.

All three orbit their host star closer than Mercury orbits the sun. Kepler-37b takes just 13 days to complete one orbit -- Mercury takes 88 days to circle the sun -- giving the newly discovered world an estimated temperature of more than 800 degrees Fahrenheit.

The other two planets are only slightly farther out, with Kepler-37c orbiting every 21 days and Kepler-37d taking 40 days to complete a circuit.

"We uncovered a planet smaller than any in our solar system orbiting one of the few stars that is both bright and quiet, where signal detection was possible," Thomas Barclay, a Kepler scientist at the Bay Area Environmental Research Institute in Sonoma, Calif., said in a NASA statement. "This discovery shows close-in planets can be smaller, as well as much larger, than planets orbiting our sun."

The findings are presented in the journal Nature.

Launched in 2009, the Kepler space telescope is equipped with a 95-megapixel camera that acts as an ultra-sensitive photometer, continually monitoring the light from more than 150,000 stars in a patch of sky in the constellation Lyra.

Planets passing in front of targeted stars cause a very slight dimming, roughly comparable to watching a flea creep across a car's headlight at night. By timing repeated cycles, computer analysis can ferret out new worlds, including potential Earth-like planets orbiting in a star's habitable zone where water can exist as a liquid.

The probability of finding sun-like stars with Earth-like planets in orbits simliar to ours - and aligned so that Kepler can "see" them - is about one-half of 1 percent. Given the sample size, however, that still leaves hundreds of potential discoveries.

To accurately measure a planet's size, however, astronomers must first know the size of the star in question. The Kepler science team determined the size of the star Kepler-37 by precisely measuring subtle flickering caused by sound waves moving through the star.

Researchers determined Kepler-37 is three quarters the size of the sun with an uncertainty of just 3 percent, a new record in the fast-moving search for exoplanets.

"Even Kepler can only detect such a tiny world around the brightest stars it observes," Jack Lissauer, a planetary scientist at NASA's Ames Research Center, said in NASA's statement. "The fact we've discovered tiny Kepler-37b suggests such little planets are common, and more planetary wonders await as we continue to gather and analyze additional data."

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